Hydraulic time constants for transpiration of loblolly pine at a free-air carbon dioxide enrichment site

The impact of stored water on estimates of transpiration from scaled sap flux measurements was assessed in mature Pinus taeda (L.) at the Duke Free-Air CO2 Enrichment (FACE) site. We used a simple hydraulic model with measurements of sap flux (J) at breast height and the base of the live crown for 26 trees over 6 months to examine the effects of elevated CO2 (eCO(2)) and fertilization (N-F) treatments, as well as temporal variation in soil moisture (M-(t)), on estimates of the hydraulic time constant (kappa). At low M-(t), there was little (< 12%) difference in kappa of different treatments. At high M-(t), differences were much greater, with kappa reductions of 27, 52 and 34% in eCO(2), N-F and eCO(2) x N-F respective to the control. Incorporating kappa with these effects into the analysis of a larger data set of previous J measurements at this site (1998-2008) improved agreement between modeled and measured values in 92% of cases. However, a simplified calibration of kappa that neglected treatment and soil moisture effects performed more dependably, improving agreement in 98% of cases. Incorporating kappa had the effect of increasing estimates of reference stomatal conductance at 1 kPa vapor pressure deficit (VPD) and saturating photosynthetic active radiation (PAR) an average of 12-14%, while increasing estimated sensitivities to VPD and PAR. A computationally efficient hydraulic model, such as the one presented here, incorporated into a hierarchical model of stomatal conductance presents a novel approach to including hydraulic time constants in estimates of stomatal responses from long-term sap flux data sets.